Abstract
Purpose
The subject of the paper is the mechatronic design of a novel robotic hand, cassino-underactuated-multifinger-hand (Ca.U.M.Ha.), along with its prototype and the experimental analysis of its grasping of soft and rigid objects with different shapes, sizes and materials. The paper aims to discuss these issues.
Design/methodology/approach
Ca.U.M.Ha. is designed with four identical underactuated fingers and an opposing thumb, all joined to a rigid palm and actuated by means of double-acting pneumatic cylinders. In particular, each underactuated finger with three phalanxes and one actuator is able to grasp cylindrical objects with different shapes and sizes, while the common electropneumatic operation of the four underactuated fingers gives an additional auto-adaptability to grasp objects with irregular shapes. Moreover, the actuating force control is allowed by a closed-loop pressure control within the pushing chambers of the pneumatic cylinders of the four underactuated fingers, because of a pair of two-way/two-position pulse-width-modulation (PWM) modulated pneumatic digital valves, which can also be operated under ON/OFF modes.
Findings
The grasping of soft and rigid objects with different shapes, sizes and materials is a very difficult task that requires a complex mechatronic design, as proposed and developed worldwide, while Ca.U.M.Ha. offers these performances through only a single ON/OFF or analogue signal.
Practical implications
Ca.U.M.Ha. could find several practical applications in industrial environments since it is characterized by a robust and low-cost mechatronic design, flexibility and easy control, which are based on the use of easy-running components.
Originality/value
Ca.U.M.Ha. shows a novel mechatronic design that is based on a robust mechanical design and an easy operation and control with high dexterity and reliability to perform a safe grasp of objects with different shapes, sizes and materials.
Keywords
Citation
Figliolini, G. and Rea, P. (2014), "Mechatronic design and experimental validation of a novel robotic hand", Industrial Robot, Vol. 41 No. 1, pp. 98-108. https://doi.org/10.1108/IR-04-2013-344
Publisher
:Emerald Group Publishing Limited
Copyright © 2014, Emerald Group Publishing Limited
Nomemclature
a=length of the rigid palm m 0
a t =theoretical length of the rigid palm m 0
b=height of the rigid palm m 0
c=length of the thumb m 4
d=thickness of each finger
e=distance between each pair of fingers
A ij =ith-revolute joint of the jth-linkage that composes the underactuated finger mechanism
I COIL =current adsorbed by the coil of valves V 1 and V 2
K T =static gain of the pressure transducer T p
m=number of elastic elements
m 0=rigid palm
m 1=proximal phalanx
m 2=median phalanx
m 3=distal phalanx
m 4=thumb
n=degrees of freedom
P A =atmospheric pressure
P OUT =outlet pressure
P S =supply pressure (six barrel)
P SET =reference pressure
r ij =link length of the jth-linkage of the finger mechanism
S 1, S 2=torsional springs acting as passive actuators
S p =end-stroke electric switches of each proximal phalanx
T=pneumatic tank
T P =pressure transducer
T PWM =period of the pulse-width-modulation (PWM)
V=volume of the operating zone at the starting configuration
V 1=2/2 solenoid-spring return valve in supply
V 2=2/2 solenoid-spring return valve in discharge
V 3=3/2 solenoid-spring return valve to open the underactuated fingers
V 4=5/2 solenoid spring-return valve to operate the thumb
V OUT =voltage feed-back signal of P OUT (0-10 V)
V PWM =PWM modulated control signal (±5 V)
V PWM1,2=PWM modulated control signals to operate V 1 and V 2, respectively, (0-24 V)
V REF =compensated voltage reference signal
V SET =voltage reference signal
α=thumb rotation angle
ɛ=error of the closed-loop control
t =maximum theoretical diameter of a cylindrical object that can be grasped
γ 1,2=angles at joints A 11 and A 12 of both triangular laminas that connect two successive linkages
γ 3=angle at joint A 42 of the distal phalanx m 3
1,2,3=counter-clockwise angles of rotation of m 1, m 2 and m 3, respectively
1M,2M,3M=maximum counter-clockwise angles of rotation of m 1, m 2 and m 3, respectively
Subscripts
i=ith-revolute joint of each jth-linkage, numbered counter-clockwise (i=1 to 4)
j=jth-linkage of the finger mechanism numbered from the palm m 0 to the median phalanx m 3 (j=1, 2)
Introduction
The mechatronic design of robotic hands is a very complex task, which involves different aspects of mechanics, actuation, and control. A suitable anthropomorphic design can be developed by referring to the human hand, which is able to grasp and manipulate objects with different sizes and shapes, but its functionality and versatility are quite difficult to mimic in terms of mechanics, actuation and control. Nevertheless, intense research activity and many prototypes have been developed and designed for prosthetic and robotic applications. Significant tendon-operated robotic hands are the Stanford/JPL and Utah/MIT prototypes, as reported in Mason and Salisbury (1985) and Jacobsen et al. (1984), respectively. These two prototypes represented for long time important reference designs to mimic the very complex functionality of the human hand. The mechanics of manipulation was also taken into account along with the grasping performance. Robotic hands can find applications as prosthetic devices for human beings, grasping devices for industrial robots, along with humanoids and walking robots (Gonzalez et al., 2011; Figliolini and Rea, 2007b). More recently, the prototype of DLR's multi-sensory articulated four-finger hand was built with all actuators and sensors integrated in the hand palm or fingers directly (Liu et al., 1999, 2008). Gifu is a robotic hand that looks very similar to the human hand, not only in terms of shape and size, but also in terms of operation and motion capabilities (Kawasaki et al., 2002; Mouri et al., 2002). The Gifu Hand II and III, like the DLR Hand, are good prototypes that have actuators inside the palm/fingers. By integrating the actuators into the hand part, the whole robot hand unit becomes compact, but it is not easy to obtain both small size and large fingertip force. The shadow dexterous hand is a robotic system that was developed by The Shadow Robot Company in London and, thus, it is commercially available with pneumatic or electric actuation (Tuffield and Elias, 2003; Kochan, 2005). This hand replicates the characteristics of the human hand as far as possible, being of similar size and shape, along with its functionality. Moreover, an interesting mechatronic design of an anthropomorphic hand with pneumatic actuation was also proposed (Raparelli et al., 2000), while a biomechatronic approach was used by Zollo et al. (2007) to the design of an anthropomorphic artificial hand that was intended for prosthetics as well as in humanoids and personal robotics. A human-like kinematic and dynamic behaviors were obtained and a special underactuated transmission was designed to keep the number of motors as low as possible while achieving a self-adaptive grasp. In fact, most of the robotic hand prototypes are very complex and expensive. An innovative design of dexterous robot hands for packaging machines was proposed by Jian Dai of King's College, London (Dubey and Dai, 2006). In particular, a dexterous reconfigurable assembly and packaging system (D-RAPS) with dexterous robot fingers was designed, built and tested for complex carton packaging. The grasper compliance and configuration optimization was analyzed by Dollar and Howe (2005).
An interesting alternative in the mechatronic design of artificial hands is obtained by using and applying the concept of “underactuation”, was proposed by Gosselin and his co-workers at Laval University, Canada, and then applied to robotic fingers pinching rigid objects with non-parallel surfaces (Montambault and Gosselin, 2001; Lalibertè et al., 2002; Birglen and Gosselin, 2004). A summary of the main results that were obtained from this research activity has been published in a reference book (Birglen et al., 2008). In addition, the control issues of a family of non-linear and underactuated systems were proposed and investigated by Arimoto (2003), while, more recently, the use of underactuation in prosthetic grasping and the optimal design of an underactuated finger mechanism were analyzed (Kyberd et al., 2011; Ceccarelli et al., 2012). Other interesting designs that make use of the concept of underactuation have been reported (Dubey and Crowder, 2011; Gabiccini et al., 2011), which deal with the hand of Southampton's WAM and the formulation of a “softly underactuated” model, respectively.
This approach was extended and applied to design the finger mechanisms of the cassino-underactuated-multifinger-hand (Ca.U.M.Ha.) at the University of Cassino and Southern Lazio, Italy, as first proposed and analyzed in Figliolini and Rea (2006, 2007a), where the overall design is outlined and preliminary experimental results are reported in terms of grasping performance without considering the further extension to the actuating force control. In particular, this research targeted a robust low-cost prototype with easy-running components and, at the same time, to mimic the performance of the human hand in terms of dexterity for grasping objects with different characteristics.
The main novelties of Ca.U.M.Ha. consist of an anthropomorphic design composed of four fingers with three phalanxes and an opposing thumb; a novel underactuated finger mechanism composed of a serial connection of a slider-rocker mechanism and two four-bar linkages, which provide one single dof because of the action of two torsional springs; pneumatic actuation of the fingers with a simultaneous operation through a unique digital signal and the additional possibility to implement a closed-loop actuating force control through a suitable PWM modulation of the analogue control signal.
The SARAH hand that probably represents the most significant prototype of underactuated robotic hand with pneumatic actuation shows a non-anthropomorphic design since composed by three underactuated fingers that are operated by a different mechanism without torsional springs to open both median and distal phalanxes, and a different pneumatic actuation without any force control.
Ca.U.M.Ha. can find several practical applications in simple industrial environments, since it is characterized by a robust and low-cost mechatronic design, compliance and simple control, which are fundamental to obtain good and safe grasping performances, rather than a high manipulability, as usually required for prosthetic applications in human beings, as well as for humanoids and personal robotics.
Mechanical design of the underactuated finger mechanism
The mechanical design of the finger mechanism is been based on the concept of underactuation applied to robotic hands. A mechanism is said to be underactuated when it has fewer actuators than dofs, i.e. underactuation allows the use of (n−m) actuators to control n dofs, where m passive elastic elements replace actuators. Thus, the concept of underactuation can be used to design a finger mechanism for robotic hands, which can envelop objects with different sizes and shapes by means of a suitable grasping sequence, which does not require an active coordination of the phalanxes.
Referring to Figure 1, the proposed finger mechanism is mainly composed of three links m 1, m 2 and m 3, which correspond to the proximal, median and distal phalanxes, respectively. A linear actuator transmits the motion to the driving link A 11 A 21 through the offset slider-rocker mechanism A 10 A 20 A 30, whose slider is connected to the moving bar of the actuator and the rocker is attached to the driving link A 11 A 21 to compose a single rigid body with three revolute joints. The four-bar linkages A 1i A 2i A 3i A 4i , for i=1, 2, are connected in series between them through the rigid body A 41 A 31 A 22, in order to transmit the motion from the driving link A 11 A 21 to both median and distal phalanxes, respectively. In particular, the rigid body A 42 A 32 P represents the distal phalanx m 3, which produces a relative motion around the revolute joint A 42 when the median or both proximal and median phalanxes are already in contact with the object to grasp, thereby becoming fixed to the palm m 0.
Under this condition, the slider-rocker mechanism A 10 A 20 A 30 along with the two four-bar linkages A 11 A 21 A 31 A 41 and A 12 A 22 A 32 A 42 become assembled to produce an eight-bar mechanism, whose driving link is A 11 A 21, with driven member the distal phalanx m 3.
The kinematic chain of this eight-bar mechanism results of type 521, since it is composed of five binary links, two ternary links and one quaternary link, as sketched in Figure 1, where three groups of four-bar linkages can be recognized. In this case, the fixed frame is the quaternary link, which is given by the palm m 0 and both proximal and median phalanxes, m 1 and m 2, respectively.
Likewise, when only the proximal phalanx is in contact with the object to grasp, both median and distal phalanxes are aligned and rotate as a unique rigid body around the revolute joint A 12=A 41 operated by the Watt-I six-bar mechanism, which consists of the slider-rocker mechanism A 10 A 20 A 30 along with the four-bar linkage A 11 A 21 A 31 A 41. Finally, when any phalanx is not in contact with the object to grasp, all three phalanxes are aligned as a single rigid body around the revolute joint A 11=A 10. The operating mechanism is now the slider-rocker mechanism A 10 A 20 A 30.
Moreover, the revolute joints A 11=A 30, A 12=A 41 and A 42, which join the palm m 0 with m 1, the proximal phalanx m 1 with m 2, and the median phalanx m 2 with m 3, respectively, are provided with suitable mechanical stoppers in order to avoid the hyper-extension and hyper-flexion of each phalanx with respect to the previous one or with respect to the palm in the case of m 1. These mechanical stoppers are very important to obtain an anthropomorphic behavior, which mimics the performance of the human index finger, and they consist of three pairs of mechanical end-strokes for each phalanx with respect to the previous one. Thus, taking into account the underactuated design of the proposed finger mechanism, both revolute joints A 12=A 41 and A 42 are provided with two passive elastic elements, in order to ensure the extended configuration of the finger mechanism before reaching the contact with the object to grasp. Consequently, during the closing motion, the underactuated finger mechanism will bend, respectively, around the revolute joint A 11=A 30 because of the action of the linear actuator, around the revolute joint A 12=A 41 when a contact will occur with m 1, and around the revolute joint A 42 when a contact will occur with m 2 or with both m 1 and m 2. In particular, this is true when the contact of m 1 and/or m 2 with the object to grasp will occur before reaching the end-stroke configurations against their mechanical stoppers, otherwise the finger mechanism will reach its fully closed and bended configuration when no contact occurs. This can be due to the small size of the object or because that particular finger is not involved in the grasping of that specific object by virtue of its irregular shape.
Therefore, referring to Figure 2, the mechanical design of the underactuated finger mechanism has been developed according to the kinematic sketch of Figure 1 and with the aim to obtain a robust and compact overall mechatronic design of the whole robotic hand. In particular, the three phalanxes have been manufactured according to a U-shape, which allows a suitable assembling of the upper transmission system that is represented by the linkage of Figure 1. The thickness of each finger is d=30 mm and each pair of them is spaced through a distance e of 15 mm.
Both torsional springs S 1 and S 2 have been integrated with the finger mechanism being coaxial with the revolute joints A 41 and A 42, respectively, in order to act as two passive elastic elements that replace two actuators, thereby reducing the number of actuators to only one from 3 dofs.
Thus, referring to Figure 3 and according to an accurate motion analysis of the human index finger, the dimensions r 11=86 mm, r 12=50 mm and r 13=46 mm have been assumed for the three links m 1, m 2 and m 3, which represent the proximal, median and distal phalanxes of the proposed underactuated mechanism, respectively. A scale factor of 2:1 has been assumed to design and build the finger mechanism with respect to the estimated average sizes of the three phalanxes of the human index finger. Moreover, the maximum counter-clockwise rotation angles 1M=83°, 2M=105° and 3M=78° have been measured referring to the fully closed configuration of the human index finger for the proximal, median and distal phalanxes, respectively. These design specifications have been necessary to design properly the mechanical stoppers of the finger mechanism with the aim to mimic the closing motion and the fully closed configuration of the human index finger.
In addition, still referring to Figures 1 and 2, the main dimensions of the rigid palm m 0 are a=95 mm and b=50 mm, while the thumb m 4 of length c=55 mm can undergo a pure rotation of angle α=30° around the revolute joint Q. In particular, the palm design was developed by considering the maximum theoretical diameter t =100 mm of a cylindrical object that can be grasped when its axis is perpendicular to the plane of motion of each underactuated finger mechanism and when the second and third phalanxes are parallel to the horizontal palm, as shown in Figure 3(b). According to this grasping configuration without the aid of the thumb, one has: Equation 1 where a t =85 mm is the theoretical length of the horizontal palm for r 12=50 mm, r 13=46 mm, r 11=86 mm and 1M=83°. The length a=95 mm, greater than a t =85 mm, was assigned in order to avoid interference. Of course, the height b=50 mm of the rigid palm m 0 is equal to one half of t .
When the diameter of the cylindrical object increases from t =100 mm and neglecting the friction forces, the object to grasp is pushed up on the right-hand side in order to reach the vertical palm while the second and third phalanxes wrap the object. The thumb will remain open when all proximal phalanxes are fully closed.
Therefore, each finger mechanism is actuated by a single double-acting pneumatic cylinder actively and by two torsional springs passively, in order to grasp objects, which can be fully enveloped by the three phalanxes; otherwise, the fully closed configuration is reached against the end-stroke mechanical stoppers.
Mechatronic design of the robotic hand
The overall Ca.U.M.Ha. mechatronic design is mainly based on the mechanical design of the proposed underactuated finger mechanism. Moreover, in order to mimic the anthropomorphic design of the human hand that is characterized by high flexibility and dexterity, but with the aim to obtain also a robust mechanical design and an easy operation and control with high reliability, along with a low-cost prototype and easy-running components, a specific and novel mechatronic design was developed with the aim to match and to satisfy the assigned design specifications.
Thus, four identical underactuated finger mechanisms have been considered to be connected to a rigid palm through four revolute joints, while a simplified opposing thumb has been designed with a unique phalanx and one dof in order to provide a supporting plate for grasping large objects.
Consequently, the exploded view of the designed Ca.U.M.Ha. with all fingers and thumb at the starting configuration is shown in Figure 4, where one has:
underactuated finger;
mechanical transmission system of the finger;
pneumatic actuator of the finger;
exploded view of a finger;
pneumatic tank;
opposing thumb;
pneumatic actuators of the thumb; and
supporting plate of the electropneumatic circuit.
This new actuation system gives an additional and effective auto-adaptability of the four underactuated finger mechanisms on objects with different shapes and sizes, because they can envelop objects showing an irregular shape not only in the plane of the motion of each finger mechanism, but also along their perpendicular direction. Thus, each finger mechanism can reach a different closing configuration according to the particular shape and size of the object to grasp, as shown in Figure 5.
In fact, the three-dimensional view of Figure 5 shows the Ca.U.M.Ha. in a generic closing configuration in which both middle and ring finger mechanisms assume two different intermediate configurations between full extension and full closing, as those of the index and the little finger mechanisms, respectively. Of course, these configurations are possible only when pertinent contacts with the object to grasp occur, because of the underactuation system of each finger mechanism.
This effective behavior of the set of four identical underactuated finger mechanisms is due to the uniform distribution of the air pressure inside the pneumatic tank and all pushing chambers of the corresponding double-acting pneumatic cylinders. In fact, during the air flow from the supply port of the electrovalve to the tank and pushing chambers, the pressure increases up to produce the closing motion of each finger, which stops when a closed configuration on the object surface is reached and, consequently, the air flow will continue to fill the chambers of the other cylinders and only when all fingers will be closed on the object surface and the pressure will be equal to the pressure supply, the air flow stops. Therefore, the particular choice of a pneumatic actuation system has been motivated by the goal to obtain this additional and effective auto-adaptability of the fingers on the object to grasp, which could not be easily given through the use of common electric drives.
The foregoing operation is produced by the electropneumatic circuit of Figure 6, which mainly consists of three parts: the actuation and sensor system of the four identical underactuated finger mechanisms (right side); the actuation system of the thumb (lower left side); and the control system of the robotic hand (upper left side).
In particular, the electropneumatic circuit of the underactuated fingers consists of the following components: four double-acting pneumatic cylinders of type Festo DSN 16-80-PPV; a manufactured pneumatic tank T; two 2/2 (two-way/two-position) solenoid-spring return valves of type SMC VQ21A1-5Y0-C6-F-Q, which are installed one in supply (V 1) and one in discharge (V 2), respectively. A 3/2 (three-way/two-position) solenoid-spring return valve V 3 of type SMC EVZ512 and a check-choke valve of type Festo GR-M5 are used, respectively, to open the underactuated fingers and to regulate the backward speed of the pistons during a generic grasping operation. The electropneumatic circuit of the thumb consists only of two double-acting pneumatic cylinders of type Festo DSN 16-80-PPV, two check-choke valves of type Festo GR-M5 and the operating 5/2 (five-way/two-position) solenoid spring-return valve V 4 of type SMC EVZ5120. Both digital control signals of the valves V 1 and V 2 are amplified at 24 V by the electronic board A likewise those of the valves V 3 and V 4 through the electronic board B.
Moreover, the four end-stroke electric switches S p are connected in series as normally-closed contacts and according to the AND logic operation of Boolean algebra. These electric switches were installed on the palm m 0 in order to detect the eventual fully closed configuration of each proximal phalanx m 1, as shown in Figure 2 in the zoom-in detail that is depicted in the dashed circle.
Therefore, the proposed robotic hand can be operated ON/OFF and through a single ON/OFF signal at 24 V by giving a good approximation of the operation of the human hand, in agreement with the design specifications. Objects with different sizes and shapes can be grasped because of the only use of a single ON/OFF electric signal, which is simultaneous with the ON/OFF command of valve V 4. In fact, the thumb is closed as default through a normally-closed switch, but it opens when, at least, one of the end-stroke electric switches S p is switched-off because of the end-stroke contact of a proximal phalanx of any of the four underactuated fingers. This is aimed to avoid any interference that may occur between the closed thumb and the distal phalanx of a finger mechanism which is performing its closing motion up to reach the object to grasp.
Actuating force control
The performance of the Ca.U.M.Ha. can be enhanced by means of suitable actuating force control, which can be obtained by controlling the air pressure inside the pushing chambers of the four pneumatic cylinders of the respective finger mechanisms. In fact, the actuating force of each mechanism is always the same because of the uniform distribution of the air pressure and the same 16 mm bore of each cylinder, but this pressure can be regulated through the proposed control system.
This new goal can be reached by means of an electropneumatic circuit similar to that described above for the ON/OFF actuation and control of the closing motion, because this pressure control system is based on the PWM modulation of the same digital valves V 1 and V 2. In fact, these valves can work as an equivalent three-way flow proportional valve to regulate the inward and outward flow-rates through the pushing chambers of the pneumatic cylinders. Therefore, this additional performance can be considered as an optional feature of the Ca.U.M.Ha. which can grasp objects of different sizes and shapes by means of a single ON/OFF signal or through a single analog signal, which allows the grasping along with the control of the actuating force. This is why the electropneumatic circuit of Figure 6 is also provided with the pressure transducer T p of type GS XPM5-10G and the electronic board A is also able to split the control signal V PWM (±5 V) and to generate the PWM modulated control signals V PWM1 and V PWM2 at 24 V. The control system has been obtained by using a common PC, provided with the electronic board of type NI-PCI6052E, along with that of type SCB 68, programmed through Lab-View.
Thus, referring to Figure 7, a specific pressure control system was designed using the PWM modulation of the pneumatic valves V 1 (supply) and V 2 (discharge). The feedback V OUT of the closed-loop pressure control is given by the pressure transducer T p , installed on the pneumatic tank T and allowing the detection of the output pressure P OUT . This pressure is compared with a reference pressure P SET and the error ɛ can be compensated through a common PID controller. The P SET signal is transformed into the V SET signal because of the static gain K T of the pressure transducer T p in order to be compared with the voltage feed-back signal V OUT .
Thus, the error ɛ is compensated by the PID controller to give the V REF analogue signal, which is transformed into the PWM modulated signal V PWM because of a suitable PWM driver with a given period of modulation T PWM . This V PWM signal shows the shape of a square-wave with an amplitude of ±5 V, which is split into two different square waves with only positive voltage and amplified up to 24 V in order to command both valves V 1 and V 2.
The static and dynamic performances of the proposed actuating force control system are strongly related to those of the digital valves V 1 and V 2, since they are identical. Thus, the attention will focus on the 2/2 (two-way/two-position) solenoid-spring return valve with pneumatic pilot and of type SMC VQ21A1-5Y0-C6-F-Q, whose exploded view is shown in Figure 8.
In particular, the actuating solenoid consisting of the iron pilot plunger 1) that represents also the armature of the magnetic circuit 2), which is energized by the solenoid coil 3) through a digital signal at 24 V that is adapted by the speed-up driver 4). The pneumatic pilot is represented by a servo-chamber consisting of an upper rigid part 5) and a membrane 6) that is assembled with the main plastic poppet 7). This poppet shows a conical axial nozzle 8), which is normally-closed by the small shutter 9) of the pilot plunger because of the action of the helical spring 10). The air flow from the inlet to the outlet port is normally-closed by the contact between the lower part of the membrane, which is assembled with the main poppet, and the support of the inner pipe 11) on the valve body. Moreover, the membrane is pushed downward by the air pressure that acts inside the servo-chamber because of the orifice 12) that is connected with the inlet port of the valve.
Thus, when the coil is not energized, the pilot plunger is pushed downward by the spring in order to close the nozzle on the main poppet, which is also pushed downward along with the membrane because of the pressure acting inside the servo-chamber. Conversely, when the coil is energized, a magnetic force attracts the pilot plunger upward by opening the nozzle and allowing the air flow from the servo-chamber toward the outlet port. Consequently, the membrane is pushed upward by opening the main poppet and allowing the air flow from the inlet to the outlet port across the inner pipe.
Thus, the mechatronic design of this electrovalve was aimed to match the best static and dynamic performances as required by the specific application on the Ca.U.M.Ha. In fact, a diameter equal to 6 mm of the inner pipe is quite big to give high conductance and flow-rate, while the pneumatic pilot system that is based on the use of a small and light plunger along with a suitable servo-chamber, gives a good dynamic performance in terms of small ON/OFF delay times, which are further reduced by the speed-up driver.
The proposed pressure control system for the actuating force control was experimentally tested in order to analyze in depth its dynamic behavior along with that of both PWM modulated digital valves V 1 and V 2. Thus, still referring to the lay-out of Figure 7, a particular time diagram for the reference pressure P SET was imposed and matched with that of P OUT by detecting also the error ɛ along with the PWM modulation of both valves. A zoom-in for the PWM modulation of valve V 1 is also shown in Figure 9 between the time range of 19.2 and 19.8 s.
In particular, a good agreement between the pressures P SET and P OUT can be observed along with the time diagram of their error ɛ that is compensated by the PID controller. The supply valve V 1 is modulated when ɛ is positive, while the discharge valve V 2 is modulated when ɛ is negative, because of the PWM driver, whose modulation period was set to T PWM =50 ms.
The speed-up behavior of each digital valve is made apparent by the experimental diagrams of Figures 10 and 11, which were obtained through a specific experimental test. In particular, the time diagrams of a voltage reference signal V REF that was assigned to be constant, the related output pressure P OUT that was detected on the tank through the pressure transducer T p , the PWM modulated voltage signal V PWM1 that was operating the supply valve V 1, and the current I COIL that was adsorbed by the coil of the electro-magnetic circuit of V 1 during its opening and closing operation, are shown simultaneously.
Thus, even if V REF is constant in this specific experimental test with the aim to give a constant average flow-rate from the supply port to the tank, the pressure P OUT does not grow up linearly according to a theoretical linear ramp, but P OUT shows several steps because of the PWM modulation of V 1. In fact, each step appears in phase with the closing time of the valve, as shown in Figure 10.
Moreover, referring to Figure 11, the time diagram of the I COIL shows a typical peak that corresponds to the opening instant of the pilot plunger before reaching the steady-state value according to Ohm's law. Nevertheless, this steady-state condition is interrupted after a short time because of the action of the speed-up driver, which increases the current value at the beginning in order to reduce the ON delay time, but this is decreased to avoid the heating of the coil and to reduce the energy consumption.
Therefore, the proposed actuating force control by means of a suitable pressure control in the pushing chambers of the pneumatic cylinders of the underactuated finger mechanisms is a possible alternative to the above-mentioned ON/OFF control of the grasping motion of the Ca.U.M.Ha. in order to improve the grasping of objects with different materials, which can be also soft and delicate to handle.
Prototype and experimental grasping tests
According to the proposed mechatronic design, a prototype of the Ca.U.M.Ha. was built and tested by means of the experimental test-bed of Figure 12, which shows:
the robotic hand prototype;
a double-acting pneumatic cylinder of the underactuated finger;
both 2/2 PWM modulated pneumatic digital valves;
the 3/2 pneumatic digital valve that operates the exhaust chambers of the pneumatic actuators of the fingers;
the 5/2 pneumatic digital valve that operates the thumb;
the external block SCB-68;
the electronic board A that splits and amplifies the signal V PWM into the signals V PWM1 and V PWM2;
the electronic board B that controls both valves V 3 and V 4; and
a PC that is supplied with a data-acquisition system of type NI PCI-6052E along with lab-view.
The total weight of Ca.U.M.Ha. is about 50 N by virtue of the wide use of aluminum and the scale factor of 2:1 with respect to the human hand, while the volume V of the parallelepiped that includes the operating zone of all fingers and thumb at the starting fully extended configuration is given by: Equation 2 which third factor of the product corresponds to the operating thickness of the hand. Thus, substituting the corresponding numerical values into equation (2), it obtains a total volume V of about 4,800 cm3.
The tank, not shown in Figure 12 because it is integrated in the palm, like the four double-acting pneumatic cylinders of the fingers, was made of steel, along with the small axle of each revolute joint. The electrovalves (3), (4) and (5) were installed on a plexiglass plate and, thus, assembled onto the rigid structure of the palm. The thumb is given by a plate that is still made of anodized aluminum, which can be rotated about point Q of Figure 1 with respect to the palm, by virtue of the two double-acting pneumatic cylinders that operate in parallel.
Thus, several experimental tests of the proposed robotic hand prototype have been carried out in order to analyze its grasping performance of soft and rigid objects with different shapes, sizes and materials.
Some of these experimental tests are shown in Figures 13-15, which refer, respectively, to the grasping of a brick and an electronic device for welding, as examples of grasp of rigid objects; the grasping of a sponge, as example of grasp of a soft object; the simultaneous grasping of objects of different shape, size and material, as example of multi-grasping operation. The support of the thumb was not required in any of these experimental grasping tests since the sizes of the objects in grasp were not big enough to open the normally-closed electric end-stroke switches S p . This means that one proximal phalanx, at least, reached the fully closed configuration to open the thumb.
In particular, Figures 13(a) and 14(b) show the case in which all fingers envelope the object in grasp by reaching the same final configuration with the only difference that the fingers are more closed in Figure 14(b) than in Figure 13(a). This behavior is due to the parallelepiped shape, quite homogeneous stiffness and symmetrical position of both brick and sponge. The non-symmetric position of the sponge and the electronic device for welding in Figures 14(a) and 13(b), respectively, give different final grasping configurations of the fingers, but the envelope of the object is ensured at same by giving a firm and safe grasp. Figure 14(b) shows the behavior of Ca.U.M.Ha. by increasing the reference pressure P SET , which produces a bigger actuation force that compresses the sponge more than in the case of Figure 14(a). Figure 15(a) and (b) show a more complex application of Ca.U.M.Ha. since referred to the simultaneous multi-grasping of several objects of different shapes, sizes and materials. In fact, Figure 15(a) refers to three peaces of aluminum, wood and plastic, while Figure 15(b) refers to four different peaces of polystyrene. Even in these cases all objects are enveloped by ensuring a firm and safe grasp.
Therefore, the prototype of the Ca.U.M.Ha. shows dexterity and reliability to perform a safe grasp of objects with different shapes, sizes and materials. In particular, the dexterity gives also the capability to perform the single grasping of objects, along with the multi-grasping of a group thereof. The safety of the grasp is ensured by the simultaneous action of the underactuated fingers, along with the opposing thumb when required. In fact, this cooperation of fingers and thumb lets to envelop the object to grasp by giving several contact points.
A possible industrial application of the Ca.U.M.Ha. robotic hand has been tested as end-effector of a binary hybrid robot with pneumatic actuation, as introduced for a leg mechanism in Figliolini et al. (2010). Referring to Figure 16, a plastic bottle with liquid inside is grasped and manipulated by performing a very common pick-and-place task and, thus, showing an example of object with irregular shape and soft material. The grasp was firm and safe at both pick-and-place tasks and during the movement of the robot within its wide workspace, which is ensured by the particular serial-parallel kinematic structure that is composed by two 3revolute-prismatic-spherical (RPS) modules. The orientation of Ca.U.M.Ha. is also given in this application by a simplified wrist, which allows the only rotation around the perpendicular axis to the moving platform.
Conclusions
The mechatronic design of a novel robotic hand, Ca.U.M.Ha., was proposed and a prtotype was experimentally tested by showing dexterity and reliability to perform a safe grasp of objects with different shapes, sizes and materials. These performance could be further investigated in next developments by means of specific performance indices, which allow the measurement of dexterity and safety of the grasp.
The main characteristics of Ca.U.M.Ha. are:
the underactuation of each finger mechanism with three phalanxes and one pneumatic actuator; and
the auto-adaptability of these finger mechanisms on objects of irregular shapes because of their capability to reach different closing configurations.
a common open-loop ON/OFF control; and
a closed-loop actuating force control because of a suitable PWM modulation of the operating digital electrovalves.
Corresponding author
Giorgio Figliolini can be contacted at: [email protected]
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